Glossary

Glossary

{"data":[{"keyValue":"A","indexValue":0,"isAvailable":true,"item":[{"title":"Abbe number (V value, or constringence)","id":"abbenumber","description":"<p>This is the ratio of refractivity over mean dispersion. The higher the constringence, the smaller the light dispersion. The Abbe number (named for the German scientist Ernst Abbe) is generally calculated with the d and e ray:<\/p>","isWithImage":false},{"title":"Aberrations, chromatic","id":"aberrationschromatic","description":"<p>Iridescence on the fringes of an image as a result of light dispersion (different optical paths according to radiation wavelengths).<\/p>","isWithImage":false},{"title":"Absorption","id":"absorption","description":"<p>The conversion of light to another form of energy during its course through the glass. The intrinsic absorption of a white (clear) glass is virtually nil.<\/p>","isWithImage":false},{"title":"Addition","id":"addition","description":"<p>Spherical power addition required to adjust distance vision to near vision provided by higher index segment in fused multifocal.<\/p>","isWithImage":false},{"title":"Annealing (See also Lehr, annealing)","id":"annealing","description":"<p>An operation consisting of heating glass blanks to the annealing temperature (in the region of 530°C for a Crown lens) and cooling them very slowly so as to reduce interior thermal stress and permit surfacing without risk of breakage.<\/p>","isWithImage":false},{"title":"Anti-reflective (AR) coating","id":"antireflective","description":"<p>AR coating is applied on the lens surface to reduce reflections and improve vision comfort. It consists&nbsp;of applying transparent thin film structures with alternate layers of contrasting refractive indices.<\/p>","isWithImage":false},{"title":"Anti-scratch coating: See Hard Coating (HC)","id":"antiscratchcoating","description":"<p>&nbsp;&nbsp;<\/p>","isWithImage":false}]},{"keyValue":"B","indexValue":1,"isAvailable":true,"item":[{"title":"Barium glasses","id":"bariumglasses","description":"<p>High-index glasses used primarily for near-vision segments in the manufacture of fused multifocals.<\/p>","isWithImage":false},{"title":"Beryl","id":"beryl","description":"<p>A natural silicate of aluminium and beryllium. In ancient times, small magnifying lenses were cut out of beryl.<\/p>","isWithImage":false},{"title":"Blanks: See Semi-finished blanks","id":"blanksseesemi","description":"<p>&nbsp;&nbsp;<\/p>","isWithImage":false},{"title":"Borosilicate glass","id":"borosilicateglass","description":"<p>Glass whose composition includes high proportions of silicon and boron. In ophthalmic optics, borosilicate glasses for example, Photogray<sup>®<\/sup>&nbsp;Extra, have special properties, viz. a lower coefficient of expansion and a lower density than other Crown glasses. They are also more chemically inert. In the consumer product sector, the Pyrex<sup>®<\/sup>&nbsp;brand name is the best-known example of borosilicate glass<\/p>","isWithImage":false},{"title":"Boss (Block)","id":"boss","description":"<p>A cylinder of low-melting-point alloy with a conical depression in the centre of the outer flat surface, or carrying a cast-iron disc with such a depression, which is attached to a lens surface to facilitate the machining of the opposite surface. Such a device is used in a prescription workshop for:<\/p><ul><li>spherical or toric generating, when located on or off centre in relation to the lens axis,<\/li><li>smoothing,<\/li><li>polishing.<\/li><\/ul>","isWithImage":false}]},{"keyValue":"C","indexValue":2,"isAvailable":true,"item":[{"title":"Composite","id":"composite","description":"<p>A product made by combining different materials. The purpose is to obtain a final product comprising a range of qualities and properties superior to those found in each material taken individually:<\/p><ul><li>organic materials: light weight,<\/li><li>mineral glasses: photochromism and scratch resistance.<\/li><\/ul>","isWithImage":false},{"title":"Composition","id":"composition","description":"<p>In glass manufacture, composition denotes the specified mixture of different vitrifiable, natural, or refined materials peculiar to each type of glass which determines its properties. It is this composition which, when mixed with cullet, is put in the furnace. Composition is also used to describe the nature and percentage of oxides which take part in the constitution of the glass.<\/p>","isWithImage":false},{"title":"Constringence (See Abbe number)","id":"constringence","description":"<p>&nbsp; &nbsp;&nbsp;<\/p>","isWithImage":false},{"title":"Crown","id":"crown","description":"<p>a. In scientific optics, a low-refractive-index, high-constringence glass which, when combined with a glass of higher index (Flint), produces chromatic lenses.<\/p><p><br>b. In ophthalmic optics, glass with a refractive index of n<sub>d<\/sub>&nbsp;= 1.523, comprising the most extensively used material.<\/p>","isWithImage":false},{"title":"Crucible","id":"crucible","description":"<p>A container made from heat-resistant materials in which glass is produced through the successive phases of melting, fining, and conditioning. Crucible melting (a manufacturing technique almost discontinued nowadays) is used only for certain kinds of special optical glass.<\/p>","isWithImage":false},{"title":"Crystal","id":"crystal","description":"<p>A white, highly refractive glass containing at least 24 percent of lead in its composition, with a sonority and particular brilliance as well as high weight for its volume.<\/p>","isWithImage":false},{"title":"Cullet","id":"cullet","description":"<p>Waste glass which is re-melted and added to the batch. Its presence facilitates melting and saves energy.<\/p>","isWithImage":false},{"title":"Curing","id":"curing","description":"<p>Heat transfer energy is used in the manufacturing of plastic lenses. When in the glass mold, the monomer resin is ready to be polymerized either by a combination of ultraviolet (UV) and thermal curing or by thermal curing alone.<br><\/p>","isWithImage":false}]},{"keyValue":"D","indexValue":3,"isAvailable":true,"item":[{"title":"Darkening","id":"darkening","description":"<p>When a photochromic lens is exposed to natural sunlight or artificial ultraviolet (UV) light, the lens appearance will darken due to the photochromic dye molecule interaction with UV light. The reverse shift is called fading.<br><\/p>","isWithImage":false},{"title":"Delivery tube","id":"deliverytube","description":"<p>The final section of the furnace where glass reaches the press at a specific viscosity, thus making possible the pressing of blanks of pre-determined shape and size.<\/p>","isWithImage":false},{"title":"Dispersion (See also: Aberrations, chromatic)","id":"dispersion","description":"<p>The phenomenon of white-light decomposition resulting from variation in the refractive index of a glass as a function of wavelength. When vision is oblique in relation to the optical axis of a lens, the prismatic effect of the off-centre rays may cause dispersion resulting in iridescence at the fringes of the image.<\/p>","isWithImage":false}]},{"keyValue":"E","indexValue":4,"isAvailable":false},{"keyValue":"F","indexValue":5,"isAvailable":true,"item":[{"title":"Fading","id":"fading","description":"<p>After returning indoors or by deactivating ultraviolet (UV) light, the appearance of a darkened photochromic lens will gradually fade back/return to its original clear state.<br><\/p>","isWithImage":false},{"title":"Fixed tint glasses","id":"fixedtintglasses","description":"<p>Fixed- (or solid-) tint glasses, as opposed to white (clear) glass, are designed to absorb some portion of visible-spectrum radiation. Generally speaking, these products are obtained by taking the composition of white glass and adding small quantities of metal oxides or rare earths such as iron, cobalt, nickel, erbium, or neodymium oxides and the like.<\/p>","isWithImage":false},{"title":"Flint","id":"flint","description":"<ul><li>In scientific optics, lead-based glass with a high refractive index and low constringence which, when associated with Crown glass, is used for producing achromatic lenses.<\/li><li>In ophthalmic optics, Flint (lead-based) glasses have been largely replaced by barium glasses for single-vision lenses segments used in fused multifocals, and by titanium glasses for single-vision lenses for high correction. These glasses provide greater constringence than lead glasses and afford considerable reduction in transverse chromatic aberrations caused by light dispersion, while also lowering the true density.<\/li><\/ul>","isWithImage":false},{"title":"Frequency of light","id":"frequencyoflight","description":"<p>Light is an electromagnetic radiation of wavelengths that are either visible or invisible to the human eye.&nbsp;<b>Visible<\/b>&nbsp;from 400 to 700 nm (<i>all the colors of the prism, from violet to red<\/i>),<br>Invisible from 10 nm to 400 nm (<i>ultra-violet light, including UVB from 280 to 315 nm and UVA from 315 to 400 nm<\/i>) and from 700nm to 1mm (<i>infrared light, divided into 3 categories A = up to 1400nm, B = up to 3000nm, C = up to 1mm<\/i>).<\/p>","isWithImage":false}]},{"keyValue":"G","indexValue":6,"isAvailable":true,"item":[{"title":"Generating","id":"generating","description":"<p>An operation which consists of grinding a spherical or toric surface onto a lens blank using a diamond-impregnated rotary tool. The surfaces obtained are precise in curvature but somewhat rough and opaque. They are subsequently smoothed and polished.<\/p>","isWithImage":false},{"title":"Glare","id":"glare","description":"<p>In general terms, glare is a strong and dazzling light. Sunglasses are the best way to reduce glare, especially polarized glasses, since annoying glare mostly comes from flat, horizontally polarized reflections which are blocked by their vertically oriented polarizing film.<br><\/p>","isWithImage":false}]},{"keyValue":"H","indexValue":7,"isAvailable":true,"item":[{"title":"Hard Coating (HC)","id":"hardcoating","description":"<p>Hard coating, abbreviated HC and also called \u201canti-scratch,\u201d is a scratch-resistant coating commonly applied on all kinds of plastic lenses. It consists&nbsp;of applying a layer (a few microns thick) of a specific resin on the lens either by dipping or spin coating.<\/p>","isWithImage":false},{"title":"High-index glasses","id":"highindexglasses","description":"<p>The optical glass industry has traditionally made a distinction between two categories of glass :<\/p><ul><li>Crowns: glasses of moderate index and high constringence,<\/li><li>Flints: glasses of high index and moderate to low constringence. Classic soda-lime glasses belong to the Crown family; so do borosilicates. They are the most frequently encountered glasses in ophthalmic optics. The use of Flints in the spectacle industry has been limited for many years by the very high density (weight) of these materials (sometimes twice as high as that of ordinary Crowns). The development of glasses containing titanium oxide (or even lanthanum oxide), which combine high index and a good level of constringence has made it possible to provide spectacle lenses with much lower density than that of traditional Flints.<\/li><\/ul>","isWithImage":false},{"title":"High index: See Mid-index and high-index lenses","id":"highindex","description":"<p>&nbsp;&nbsp;<\/p>","isWithImage":false}]},{"keyValue":"I","indexValue":8,"isAvailable":true,"item":[{"title":"In-mass: See Photochromic in-mass technology","id":"inmass","description":"<p>&nbsp;&nbsp;<\/p>","isWithImage":false},{"title":"Isotropism","id":"isotropism","description":"<p>The state of bodies whose physical properties are identical in all directions. Amorphous materials are, generally speaking, isotropic, whereas crystalline matter is normally anisotropic (its physical properties depend on the direction in which they are measured).<\/p>","isWithImage":false}]},{"keyValue":"J","indexValue":9,"isAvailable":false},{"keyValue":"K","indexValue":10,"isAvailable":false},{"keyValue":"L","indexValue":11,"isAvailable":true,"item":[{"title":"Lanthanum glass (See High-index glasses)","id":"lanthanumglass","description":"<p>&nbsp;&nbsp;<\/p>","isWithImage":false},{"title":"Lead glasses (See Crystal, Flint)","id":"leadglasses","description":"<p>&nbsp; &nbsp;<\/p>","isWithImage":false},{"title":"Lehr, annealing","id":"lehrannealing","description":"<p>Equipment for the continuous processing treatment of lens blanks at the so-called annealing temperature (in the region of 500 to 700° C). The purpose of annealing is to reduce thermal stress within the blanks so that they can be surfaced without risk of breakage. In the special case of photochromic blanks, the purpose of heat treatment carried out concurrently with annealing is to develop silver-halide crystals within the blanks. It requires temperature regulation in different zones of the lehr with an accuracy to within 1° Celsius.<\/p>","isWithImage":false},{"title":"Linear thermal expansion coefficient","id":"linearthermal","description":"<p>A measurement to express the lengthening of a glass sample per unit of length for a variation of 1°C within a defined temperature range (from 25 to 300°C for spectacle lenses). The lower the coefficient, the more the glass is likely to resist thermal shocks without breakage.<\/p><p>In ophthalmic optics, this characteristic can be of great importance when it comes to high-temperature treatment (for example, tempering).<\/p>","isWithImage":false}]},{"keyValue":"M","indexValue":12,"isAvailable":true,"item":[{"title":"Mid-index and high-index lenses","id":"midindex","description":"<p>Refractive indices up to 1.56 are generally described as \u201cmid index\u201d and those above are \u201chigh indices\u201d. Plastic materials (such as nylon, polycarbonate, Sunsensors®, \u2026) can be found within the 1.5 \u2013 1.71 range of indices, whereas glass reaches 1.9. Due to their lighter weight and thinner-profile advantage over low-index lenses (n = 1.50), mid- and high-index lenses are increasingly demanded by ophthalmic lens wearers.<br><\/p>","isWithImage":false},{"title":"Mineral glass","id":"mineralglass","description":"<p>Opposite of organic glass; obtained from silica sand with a high content of SiO<sub>2<\/sub>&nbsp;(silica=quartz) and a low level of impurities.<br><\/p>","isWithImage":false},{"title":"Mold","id":"mold","description":"<p>In ophthalmic glass making, the tooling on the press table which collects the gob of hot glass which is then pressed to form a lens blank, or «moulding». The edge of the mould («ring») produces the cylindrical shape, and the bottom of the mould («valve») forms the convex face of the moulding.<\/p>","isWithImage":false},{"title":"Molding (pressing or «blank»)","id":"molding","description":"<p>A raw glass disc or similar shape obtained by pressing a hot glass gob which will later be processed into a spectacle lens to meet prescription requirements. The curves of mouldings destined for mass-production machining are formed as closely as possible to the curves of the finished lens.<\/p>","isWithImage":false},{"title":"Monomers - Polymers","id":"monomerspolymers","description":"<p>In chemical terms, a monomer is an organic compound whose molecules can join together to form three-dimensional networks of polymer chains, thanks to a reaction called \u201cpolymerization.\u201d Polymers are large molecules, more complex than monomers and with much more diverse shapes and properties, made up of many simple repeated units. Specific types of monomers and polymerization reactions are used to manufacture plastic lenses.<\/p>","isWithImage":false}]},{"keyValue":"N","indexValue":13,"isAvailable":true,"item":[{"title":"Nylon","id":"nylon","description":"<p>Chemical name: diallylglycol carbonate monomer. Plastic material used for both ophthalmic frame and lens manufacturing, via very specific manufacturing processes<\/p>","isWithImage":false}]},{"keyValue":"O","indexValue":14,"isAvailable":true,"item":[{"title":"OHSAS (Occupational Health and Safety Assessment","id":"ohsas","description":"<p>As defined by British Standards International (BSI):<br>The OHSAS specification gives requirements for an occupational health and safety (OH&amp;S) management system, to enable an organization to control its OH&amp;S risks and improve its performance. It does not state specific OH&amp;S performance criteria, nor does it give detailed specifications for the design of a management system.<\/p><p>OHSAS 18001 has been developed to be compatible with the ISO 9001 (Quality) and ISO 14001 (Environmental) management systems standards, in order to facilitate the integration of quality, environmental, and occupational health and safety management systems by organizations.<\/p>","isWithImage":false},{"title":"Organic compound","id":"organiccompound","description":"<p>In chemical terms, a compound containing hydrocarbon groups. Manufacturing ophthalmic plastic lenses mainly relies on organic chemistry. (Manufacturing ophthalmic glass lenses mainly relies on mineral chemistry).<\/p>","isWithImage":false},{"title":"Organic glass","id":"organicglass","description":"<p><i>Opposite of mineral glass<\/i>.<\/p><p>Transparent plastic material made from organic chemistry (monomers/ resins/ catalysts/ polymerization), which has several common properties with glass; this is why the ophthalmic market relies on both categories to address diverse customers\u2019 needs and expectations. Plastic lenses are less scratch-resistant than glass lenses (thus requiring Hard Coating), not as perfectly transparent, but they are much lighter and some of them are highly resistant to impact.<\/p>","isWithImage":false}]},{"keyValue":"P","indexValue":15,"isAvailable":true,"item":[{"title":"Parison","id":"parison","description":"<p>A drop, or gob, of glass which is cut by shears as it leaves the delivery tube. The parison then falls into the mould to be pressed into a lens blank. The weight of the parison corresponds exactly to the weight of the pressed blank.<\/p>","isWithImage":false},{"title":"Photochromic dye","id":"photochromicdye","description":"<p>The photochromic dye is an organic molecule which undergoes a reversible chemical reaction when exposed to ultraviolet A (UVA): the opening or closing of its structure, respectively causing the fading or darkening of the lens. Most of the time, a photochromic plastic lens will incorporate a specific mix of several dyes, all of them photochromic.<br><\/p>","isWithImage":false},{"title":"Photochromic in-mass technology","id":"photochromic","description":"<p>The in-mass photochromic technology disperses photochromic dyes throughout the lens material with a specific manufacturing process ensuring that they are distributed evenly. As ultraviolet (UV) exposure may, in the long run, degrade some of these molecules (those closer to the convex curve), others as yet inactivated replace them, providing a long-lasting, consistent color over the life of the prescription.<br><\/p>","isWithImage":false},{"title":"Photochromic plastic resin","id":"photochromicplasticresin","description":"<p>Photochromic resin is a mixture of photochromic dyes in a liquid monomer resin.<br><\/p>","isWithImage":false},{"title":"Photochromism (photochromicity)","id":"photochromism","description":"<p>Photochromism may be defined as the modification by solar radiation of a material's light absorption characteristics. Thus, certain glazed articles are susceptible to a slight darkening after some years of exposure.<\/p><p>In general, this phenomenon is not reversible. In photographs, for example, sensitive particles undergo a definitive transformation on exposure to radiation in the near-ultraviolet portion of the spectrum. The invention of photochromic glass, on the other hand, has introduced a family of products in which the active elements, silver halide particles, are capable of undergoing a succession of darkening / clearing cycles without evidence of fatigue as manifested by a slowdown in their light response capacity. A representation of the elementary mechanism involved corresponds to the following electronic exchange between the silver atoms and their immediate environment.<\/p><p><br>Equilibrium is moved towards the right under the effect of ultraviolet or short-wavelength visible radiations. In the absence of this type of radiation, the system tends to revert to its original state.<\/p>","isWithImage":false},{"title":"Plastic lenses","id":"plasticlenses","description":"<p>Plastic lens raw materials can be divided in two groups: thermosetting and thermoplastic resins.<\/p><ul><li>Thermosetting resins, such as nylon, medium &amp; high index lens resins, are hardened by heating without any possibility of reshaping. This type of resin is transformed into lens by casting.<\/li><li>Thermoplastic resins, such as polycarbonate, soften when heated and can thus be reshaped at specific temperatures. This type of liquid resin is transformed into lens by injection.<\/li><\/ul>","isWithImage":false},{"title":"Platinum","id":"platinum","description":"<p>A noble metal used in those parts of the furnace which are heated to the highest temperatures. Platinum is used for its heat-resistant and non-polluting properties, and for its chemical inertia.<\/p>","isWithImage":false},{"title":"Plunger","id":"plunger","description":"<p>That part of the tooling fixed to the piston rod which presses the parison in the mould. The shape of the plunger determines the curvature of the concave face.<\/p>","isWithImage":false},{"title":"Poise (See also Viscosity)","id":"poise","description":"<p>A poise is the unit of measurement of the dynamic viscosity of a liquid manifesting a resistance of 1 dyne when sliding on a flat surface of 1 sq. cm. with an increasing speed of 1 cm per second / per cm.<\/p>","isWithImage":false},{"title":"Polarized glass","id":"polarizedglass","description":"<p>In simple terms, polarized sunglasses eliminate most of the annoying glare coming from flat reflective surfaces (roads, snow, lawns, lake water,\u2026). This effect is obtained by the presence of a polarizing film between two thin layers of glass, one being a tinted glass (external side, with all necessary sunglass treatments), and the other one being a neutral glass.<br><\/p>","isWithImage":false},{"title":"Polishing","id":"polishing","description":"<p>The treatment of a ground lens surface (see smoothing) to make it absolutely transparent. Polishing occurs by rubbing on a tool covered with felt or a plastic sheet while the lens is constantly sprayed with water containing finely powdered cerium oxide in suspension.<\/p><p>This operation should in no way alter the curves of the lens previously generated and smoothed.<\/p>","isWithImage":false},{"title":"Polycarbonate lenses","id":"polycarbonatelenses","description":"<p>Polycarbonate is one among several plastic materials used to manufacture ophthalmic lenses.<\/p><p>Its main benefits for patients are its&nbsp;excellent impact resistance, light weight, and high refractive index (n= 1.59). Its market share is quite significant in the USA.<\/p><p>Polycarbonate is a thermoplastic polymer. Such lenses are made by injection molding. The adequate lens forming technology consists&nbsp;of melting the raw material granules by heating, then injecting into metallic or glass molds at high temperature for a few seconds.<\/p>","isWithImage":false}]},{"keyValue":"Q","indexValue":16,"isAvailable":false},{"keyValue":"R","indexValue":17,"isAvailable":true,"item":[{"title":"Refractive index","id":"refractiveindex","description":"<p>This is the quotient of the speed of monochromatic light propagated in air to the speed propagation of the same light in any other medium. In ophthalmic optics, refractive indices are generally given for two wavelengths :ne : 546.1 nm (Mercury line) green,nd : 587.6 nm (Helium line) yellow.<\/p>","isWithImage":false}]},{"keyValue":"S","indexValue":18,"isAvailable":true,"item":[{"title":"Shears","id":"shears","description":"<p>In the context of glass-making, mechanical cutters equipped with special alloy blades to cut the glass as it leaves the delivery tube so as to obtain a repeated series of very accurately weighted parisons for pressing in moulds.<\/p>","isWithImage":false},{"title":"Silver halide","id":"silverhalide","description":"<p>Silver halides are compounds blending silver with elements which are halogens, such as chlorine or bromine. In their pure state they correspond to the formulae AgCl and AgBr. When added to a glass composition, silver halides are dispersed throughout the substance of the material as minute particles (about 100 angstroms). The number and size of these particles is determined by the application of thermal treatment, to ensure the reproducibility of the photochromic glass materials transmission characteristics and colour (gray or brown).<\/p>","isWithImage":false},{"title":"Smoothing (fine grinding)","id":"smoothing","description":"<p>This operation is designed to refine the rough surface of the lens following the process of generation. Smoothing is achieved by mechanical rubbing of the lens surface on a cast-iron counterform, while the lens is sprayed with a solution of emery in water suspension. Lens smoothing can also be effected using a counterform impregnated with small pellets of diamond sunk into a sintered bronze matrix. Smoothing should not alter the lens curve previously generated.<\/p>","isWithImage":false},{"title":"Soda-lime glasses","id":"sodalimeglasses","description":"<p>These are the most prevalent glass types (used for window glass, tableware, and so on). In addition to silicon, which accounts for about 70 percent&nbsp;of their composition, these glasses also contain sodium and calcium oxides. For spectacle lenses, ordinary Crowns belong to this category of glass. They differ essentially according to the raw materials selected and the choice of additional ingredients to determine their optical and physical properties.<\/p>","isWithImage":false},{"title":"Substrate","id":"substrate","description":"<p>The primary material or the underlying layer on which other materials (such as a coating, a paint, or a treatment) is applied.<\/p>","isWithImage":false}]},{"keyValue":"T","indexValue":19,"isAvailable":true,"item":[{"title":"Titanium glasses (See High Index glasses)","id":"titaniumglasses","description":"<p>&nbsp;&nbsp;<\/p>","isWithImage":false},{"title":"Top-siding (of a fused bifocal lens)","id":"topsiding","description":"<p>An operation which consists of grinding away all the surplus thickness of the near vision segment fused onto the «major» lens. After top-siding, the whole of the convex face of the rough bifocal lens becomes a continuous spherical surface on which, because of the difference in material index, the exterior contour of the near vision segment can be seen.<\/p>","isWithImage":false},{"title":"Torr","id":"torr","description":"<p>Unit of pressure adapted for measuring high vacuums and derived from the name of the Italian physicist TORRICELLI.<\/p><p>1 Torr = 1 mm of mercury<\/p><p>Units for surface treatment through vacuum depositing operate under residual pressures of 10-4 to 10-6 Torr.<\/p>","isWithImage":false},{"title":"Transmission","id":"transmission","description":"<p>For a given wavelength, this is the quotient of transmitted light flux Φt, to the incident light flux Φi<\/p><p>The transmission factor, spectral transmittance, varies with wavelength. The variation of this factor as a function of wavelength is represented by the transmission curve.<\/p><p>The difference in values of transmitted light flux and incident light flux are caused by:<\/p><ul><li>the intrinsic absorption of glass (this is negligible for certain lenses such as clear (white) lenses),<\/li><li>a loss by reflection on both sides of air / lens diopter (see the chapter on «Anti-reflection and tint coatings»).<\/li><\/ul>","isWithImage":false},{"title":"True density","id":"truedensity","description":"<p>This is the quotient between the mass and the volume of the glass. True density is expressed in terms of grams / cubic centimetres. For glass spectacle lenses density can vary from 2.4 to 4.0 g/cm<sup>3<\/sup>.<\/p>","isWithImage":false}]},{"keyValue":"U","indexValue":20,"isAvailable":false},{"keyValue":"V","indexValue":21,"isAvailable":true,"item":[{"title":"Viscosity","id":"viscosity","description":"<p>Viscosity may be defined in terms of the internal friction of a liquid (the unit of measurement is the poise). Bear in mind that a perfect fluid would have a zero viscosity and that a perfect solid would have an infinite viscosity reading. The chart gives characteristic viscosity values for the manufacture of glasses and explains what they mean.<\/p>","isWithImage":false},{"title":"Vitroceramics - Glass ceramics","id":"vitroceramicsglassceramics","description":"<p>Vitroceramic objects are made using glass manufacturing techniques. After shaping, they are subjected to heat treatment which develops crystals within the amorphous glass matter. Vitroceramics thus achieve, generally speaking, much greater mechanical strength and heat resistance than the glass used to make them. Certain ceramics have a zero coefficient of expansion. In the consumer product sector, the Vision<sup>®<\/sup>&nbsp;brand named articles are a well-known example of vitroceramics.<\/p>","isWithImage":false}]},{"keyValue":"W","indexValue":22,"isAvailable":true,"item":[{"title":"Wavelength (See Frequency of light)","id":"wavelength","description":"<p>&nbsp;&nbsp;<\/p>","isWithImage":false}]},{"keyValue":"X","indexValue":23,"isAvailable":false},{"keyValue":"Y","indexValue":24,"isAvailable":false},{"keyValue":"Z","indexValue":25,"isAvailable":false},{"keyValue":"«","indexValue":26,"isAvailable":true,"item":[{"title":"«Guinandage» - Stirring (after its inventor Mr.P.L","id":"guinandage","description":"<p>The operation of mechanically mixing a mass of viscous liquid glass before it arrives in the delivery tube, in such a way as to obtain glass with optimum homogeneity.<\/p>","isWithImage":false}]}]}